1. Field of the Invention
This invention generally relates to optical systems and more specifically to endoscopes and the design of objective and related optical systems used in such endoscopes.
2. Description of Related Art
Endoscopes have obtained great acceptance within the medical community in connection with a number of procedures primarily because they provide a means for performing procedures with minimal patient trauma by enabling a physician to view directly the internal anatomy of a patient. Over the years a number of endoscopes have been developed and have been categorized according to specific applications. Many have specific names including arthroscopes, cystoscopes, proctoscopes, laparoscopes.
Generally, in whatever form, an endoscope comprises an objective lens system at the distal end of the endoscope that forms an image of an object within a patient in some environmental media such as air, water, a saline solution or the like. An eyepiece or ocular system at the proximal end presents the image for viewing visually, electronically or otherwise externally of a patient. An image transfer system intermediate the objective and the eyepiece systems transfers the image produced by the objective lens system to the eyepiece system.
Significant efforts have been undertaken to improve the optical designs of these endoscopes with attention at various times directed to individual ones of the constituent optical systems. However, as known, a number of diverse optical elements introduce various aberrations, distortions and other optical problems that can affect the quality of the image which the physician views. Moreover, the presence of these problems and attempts to compensate for them or cancel them complicate the design process. The design process is further complicated because the number of variables available to an optical designer for designing each of the objective, image transfer and eyepiece systems individually and collectively is generally less than required to individually compensate each source of optical problem. Consequently a final design normally is what the optical designer considers a reasonable compromise for a particular application.
For example, U.S. Pat. No. 5,175,650 to Takayama et al. discloses an objective lens system designed for an endoscope having a short total length, a small outside diameter and a wide field angle. According to this design, the objective lens system includes a front negative lens unit, an aperture stop, a positive lens unit and an infrared cut filter. This system allows rays to be incident on the filter at heights lower than the outside circumference of the filter thereby to correct certain distortions inherent in such an endoscope.
U.S. Pat. No. 5,327,283 to Zobel discloses another optical system for an endoscope with an objective with an object-side negative lens cluster to reduce image scale defects, such as distortion and field curvature. In this objective the object-side negative lens cluster comprises two menisci each having a negative refractive power to reduce the defects of distortion and to produce a flattened image field.
As another example, U.S. Pat. No. 5,416,638 to Broome discloses an endoscope that includes a transfer module, or relay lens unit, an objective element and an eyepiece, or ocular element. The objective element includes a distal glass window having a plano-spherical shape or plano-aspherical shape, a prism and a plurality of lenses all of which produce an image for transfer through the transfer module to the eyepiece. This particular design is adapted to minimize adverse effects of certain sources of aberration by transferring residual aberration in the image formed by the objective element to the ocular element for correction.
As previously indicated, the distal end of an endoscope is introduced into and objects viewed through a wide variety of environmental media. Initially a physician may test the endoscope by viewing an object in air. In actual use the physician may view an object while the distal end of the endoscope is immersed in a fluid, such as water, saline solutions, air or carbon dioxide. Conventionally an optical designer may optimize the design for viewing in a fluid characterized by a particular index of refraction and thereby accept any distortion and other aberrations that might occur when imaging an object in a fluid having a different index of refraction. U.S. Pat. No. 5,424,877 to Tsuyuki et al. discloses such a design that is adapted for allowing the observation of objects located in liquids. Each disclosed embodiment includes a window with a distal most surface having either a planar or aspherical surface.
Taken collectively, the foregoing references disclose various objective lens system optical designs for meeting some particular design criterion. In each, however, the designs involve a window at the distal-most position that defines either a planar or aspherical distal most surface. Windows with aspherical surfaces generally have not been used in practical systems even though the use of one or more aspherical surfaces would introduce another set of multiple lens design variables. Those optical systems that incorporate aspherical surfaces apparently do so for reasons other than controlling distortion.
U.S. Pat. No. 4,805,598 to Ueda discloses an optical system with lenses and a highly-viscous gel-like substance that is free from any water vapor. Ueda's primary objective was to produce an endoscope that resists clouding. Ueda, in one embodiment, discloses a distal-most objective lens element with an exterior radiused convex surface with a concave central surface on the proximal side. A series of another proximally facing spherical lens, biconcave lens and biconvex lens produce an image. In another embodiment, Ueda discloses the double spherical lens located behind a distal window. A fiberoptic or relay lens image transfer section produces an image at an eyepiece or CCD device.
U.S. Pat. No. 5,547,457 to Tsuyuki et al. discloses eight embodiments of objective lens systems adapted to be interchangeable on the distal end of an endoscope as the need arises to view objects. A distal most lens has a convex distal surface and is a negative lens. The second lens is a positive lens to form a real image. It is stated that the introduction of the negative lens allows the final image to be flatter and allows greater correction of aberrations at long and short distances. A second of these embodiments incorporates a distal most spherical lens surface. Other embodiments include planar distal most lens surfaces. Tsuyuki et al. also disclose that the distortion at the full field of view can approach 50%.
Great Britain Patent No. 1,157,932 discloses an endoscope in which two lenses form an inverted teleplate objective lens.
None of the Ueda, Tsuyuki et al. or Great Britain patents describes or suggests any control over the distortion that the lens systems introduce into the final image. Each seems to have designed an optical system with paramount interest in the control of other parameters, all to the detriment of distortion, particularly in the case of the lens systems that the Tsuyuki et al. patent discloses.
Indeed, commercially available endoscopes historically have been manufactured and continue to be manufactured with windows that have a planar surface exposed to the environment and the designers of endoscopes continue to develop lens systems without the additional degree of freedom that a non-planar surface would introduce even summarily accepting changes in distortion and other aberrations that can result when an object is viewed through different environmental media.